the last part is wrong, it should be rebase, Sylius already has some documentation that was merged days ago on that. It is the same almost as the symfony one. In my opinion if you are also starting or want to heavily contribute to any project check Gush. http://github.com/gushphp/gush you will never regret it and I offer 100% support to its usage :)
This question of yours is particularly focused on Sylius, I think i talked about rebasing on the earlier versions of the book you have. I personally rebase even my own projects but i try to keep the customization to a minimum of commits so that i can rebase them well and quickly to upgrade to several versions later.
Other than that just check Sylius documentation on contributions.